Locating Nuclear Material by Gamma Scanning

For detection and identification of nuclear material we equipped a transportable container with neutron and gamma measurement systems. The container itself provides the necessary infrastructure, e.g., energy supply, communication means, and work places. A gamma scanner of type RoSCAN complements the abilities of the system. A gamma scan can locate the nuclear material in an object of unknown inner structure and thus help to identify its type, e.g., nuclear or radiological. The gamma scanner is equipped with two different detectors. A CZT room temperature semi conductor detector is used if high resolution is crucial. A CsI scintillation detector with less energy resolution but more efficiency is used when a short measuring time is required. The CZT spectrum can also be used to run computer codes like MGA or MGAU for the determination of the isotopic composition of nuclear material. An optical (black and white) picture of the measurement situation is taken before the gamma measurement starts. The gamma scanning area is divided into minimum 9 and maximum 1728 fields and the fields are measured sequentially. After completion of the scan the measurement integral of each field is superimposed in different colors proportional to the gamma intensity on the optical picture. In addition, an energy spectrum can be displayed for each field. The RoSCAN can be controlled from a distance of up to 80 meters via cable. We have investigated in the possibility of locating nuclear material with this gamma scanner. Therefore, we performed measurements with different point sources at various distances and with different measuring times. We also applied different shielding to account for surrounding material and containers. Results of our measurements and limits of the system will be presented. The measurements show that gamma scanning may help to prohibit nuclear proliferation and nuclear terrorism.